A digital multifunction peripheral includes, for example, an engine control unit, a laser unit, a developing device, and a fixing device. The engine control unit controls the laser unit, the developing device, and the fixing device. The laser unit forms an electrostatic latent image. The developing device develops the electrostatic latent image. The fixing device fixes the developed image on a sheet. The fixing device presses and heats the sheet in order to fix the image on the sheet. The fixing device may be heated to temperature equal to or higher than 200° C. The engine control unit has a safety function for preventing the temperature of the fixing device from rising to abnormally high temperature even when runaway of a control program or a failure of components occurs. The safety function of the fixing device is established by, for example, an electronic circuit (a safety circuit) in order to realize the safety function without the intervention of software. The safety circuit monitors the temperature in the fixing device using a thermistor or a thermopile.
However, in the digital multifunction peripheral, power supply voltage may be unstable, for example, immediately after a power supply is turned on or immediately after the power supply is turned off. When the power supply voltage is unstable, it may be difficult for the safety circuit to stably operate. When the power supply voltage is unstable, two problems explained below are likely to occur in the safety circuit.
A first problem is that a relation between reference voltage and output voltage of the thermopile or the like is likely to be unstable. The reference voltage is often generated by resistance-dividing the power supply voltage. This means that, when the power supply voltage becomes unstable, the reference voltage also becomes unstable. A temperature detection module such as the thermopile requires time of about several tens milliseconds to several seconds for stabilization of an output value (a return value) of a sensor immediately after the power supply is turned on and has a function of retaining the output value in order to reduce the influence due to a change in the power supply voltage. In short, when the power supply is unstable, a magnitude relation between the reference voltage susceptible to a change in the power supply voltage and detection voltage robust against a change in the power supply voltage tends to be opposite to an actual magnitude relation. This means that the safety circuit is liable to malfunction. It is also conceivable to generate the reference voltage using a Zener diode or the like. However, since there is a problem in practice in an error range or the like of the Zener diode itself, the Zener diode is not suitable as an element for generating the reference voltage.
A second problem is that plural power supply systems are provided in the safety circuit. A comparator IC (a comparator) can perform accurate comparison only at a voltage level substantially lower than power supply voltage for the IC. Therefore, the comparator cannot use power supply voltage same as that for the thermopile as driving voltage. For example, when the thermopile is 5V-driven, the comparator IC needs to be driven at voltage equal to or higher than 6.5V (e.g., 12V). In this case, if 12V (the driving voltage for the comparator) falls earlier at a stage when 5V (the driving voltage for the safety circuit) still sufficiently remains after the power supply is turned off, since it is not guaranteed that output of the comparator is normal, malfunction tends to be caused.